1. Field of the Invention
Applicant's invention relates generally to the field of refrigeration, and more particularly but not by way of limitation, to a method and apparatus for sequentially defrosting a series of evaporators using hot refrigeration gas (also referred to herein as high pressure gas).
2. Discussion
Refrigerated display cases are common to grocery stores, convenience stores, and other purveyors of refrigerated or frozen foods. The display cases are frequently located in the same general location within the store. For ease of installation and convenience to shoppers, the display cases are commonly arranged to form a contiguous line or in series of cases. Adjacent display cases often share similar refrigeration demands. Within each case, a fan circulates cold air in a duct that encircles the case. The duct encloses an evaporator of a refrigeration system.
Grocery stores display frozen foods in open horizontal cases, closed horizontal cases wherein the frozen food products are accessible through glass doors, and closed vertical cases wherein the frozen food products are accessible through glass doors.
Grocery stores may display milk and related products in walk-in refrigerators. The food products are accessible through glass doors, and the shelves are re-stocked from within the walk-in refrigerator. Cheese, cold cuts, butter, juices, refrigerated desserts, and similar items may be available from tiered open-front cases.
In most instances, each case, whether used for refrigerated foods or frozen foods, contains a single evaporator. When appropriate, however, a cold case may contain two, three, or even more evaporators. Here, as in the refrigeration industry, the term “evaporator” may be used interchangeably with the term “evaporator coil” or “cooling coil,” from time to time, to mean the evaporator of a refrigeration system where environmental cooling occurs. The term “cold case,” as used herein, includes all types, styles, and configurations of refrigerated food cases.
Whether the cold case is an open horizontal frozen food display case or a walk-in dairy case, and whether the cold case has one or more cooling coils, every cold case faces a common problem. Over time, the circulating cold air entrains water vapor from the ambient air. The entrained water vapor condenses and freezes on the cold evaporator coil, thereby decreasing heat transfer efficiency between the refrigerant in the evaporator coil and the air in the duct. Each evaporator must be defrosted periodically to remove the frozen condensate. Thus, each refrigerated case has a refrigeration cycle of operation (also referred to herein as a refrigeration mode) and a defrost cycle of operation (also referred to sometimes herein as a defrost mode). During the refrigeration cycle, the refrigeration system cools the case. During the defrost cycle, a heat source melts frozen condensation which has collected on the evaporator coil.
A metering device introduces high pressure liquid refrigerant into a distributor which, in turn, distributes the refrigerant to the evaporator coils to cool the circulating air. The refrigerant within the evaporator absorbs heat from the circulating cold air used to cool the refrigerated display case. As the refrigerant absorbs heat, the refrigerant changes from a low pressure liquid to a low pressure vapor and then, on further absorption of heat, the temperature of the low pressure vapor increases. The terms “low pressure vapor” and “low pressure gas” are used interchangeably to describe the gaseous refrigerant as it leaves the evaporator after absorbing heat from the air duct. Low pressure vapor streams from two or more evaporator suction lines are combined in a low pressure vapor header (also referred to, interchangeably, as a “low pressure vapor suction header” or “suction header”) from which the refrigerant compressor takes suction.
The compressor compresses the low pressure vapor to a high pressure vapor, also referred to herein interchangeably as “high pressure vapor,” “high pressure gas” (HPG) or “hot gas”. A heat exchanger, normally referred to as a condenser because of its function, then cools the high pressure vapor sufficiently to change the high pressure vapor (HPV) refrigerant to a high pressure liquid (HPL). The high pressure liquid refrigerant is collected in a liquid receiver. From the liquid receiver, the high pressure liquid refrigerant is piped through a high pressure liquid refrigerant header to distributors. A metering device controls introduction of the high pressure liquid refrigerant into the distributor. Automatic expansion valves (commonly referred to as AEV or AXV valves), thermal expansion valves (commonly referred to as TEV or TXV valves), capillary tubing, and simple orifices are all known in the art as devices capable of metering the high pressure liquid refrigerant into the evaporator distributor. In some cases, high pressure liquid refrigerant is metered based on the temperature of the low pressure vapor leaving the evaporator. A single evaporator normally includes several branches which receive refrigerant from a common distributor.
A typical supermarket may have as many as 100 cold cases containing one or more evaporators within each case. The cold cases are typically arranged in groups. Long horizontal cases with open tops may be arranged end to end to permit access from both sides. Walk-in cases are often arranged side-by-side for shopping convenience. Whether electric heating or hot gas heating is used to defrost the cold cases, operators avoid defrosting all cases simultaneously. Instead, the cold cases are grouped based on the build-up of frost within the cases. Those cases which accumulate frost rapidly may be defrosted as many as four times in a 24-hour period. Other cases may require defrosting only three times per day, twice a day, or once a day. Some cases hold frozen foods, while other cases (e.g., dairy cases) require only moderate refrigeration. Still other cases (e.g., a cold case used to hold fresh flowers) my require only minimal refrigeration. The grouping of cases for defrosting may combine cases of differing refrigeration requirements. As used herein, the term “group” is used to mean at least two evaporators which share a common compressor suction header (also called the “evaporator suction line” herein) and a common high pressure liquid header from the condenser.
It is currently common practice to defrost the evaporator coils in a series of cases at the same time, in part because contiguous refrigerated display cases often share a common defrost timer. It is also common to defrost the evaporator coils every 6-8 hours. There are several notable problems with this approach to defrosting the evaporator coils of several cases at the same time.
One problem is that defroster units of the existing art generate a lot of water vapor during a defrost cycle. If a line of contiguous cases is defrosted at the same time, an undesirable layer of frost may accumulate within the case.
Another problem caused by defrosting the cases at the same time is the need for greater electrical power at the same time. Because the defroster unit wiring is often on the same circuit for a given series of cases, this in turn causes a need for larger wiring sizes to carry the high current demand required for the defrost cycle. Additionally, because the cost of power from public utilities is often based on peak demands, the cost of power may be greatly increased by defrosting all the cases at the same time.
Yet another problem with defrost control systems of the existing art is that many are highly complex with digital components and programmable controllers. This makes repairs difficult for repairmen of ordinary skill in the refrigeration art, who are often only familiar with non-digital electrical components. The term “non-digital” refers to relays, contactors, sensors, coils, switches and any other component that generally does not process digital information.
One of the most expensive aspects of the existing practice of defrosting a series of contiguous cases at the same time is that it often leads to food spoilage. By shutting down the refrigeration cycles of contiguous cases at the same time, there can be an increase in the temperature of the food product in the cases. Also, there is often a greater increase in the display section temperature of each case due to the combined effect of defrosting several contiguous cases at the same time.
The applicant recognized a need for an improved method and apparatus for defrosting refrigerated display cases to avoid the problems created when refrigerated display cases are simultaneously defrosted and also to avoid the problems of having complex digital components. Thus applicant obtained U.S. Pat. No. 6,629,422 for a Sequential Defrosting of Refrigerated Display Cases using electric heaters. In the '422 patent, applicant disclosed and claimed a time-initiated, time-terminated defrost control method using electric heat defrosting. Electric heaters apply heat to the outside portions of the evaporator coils and to the heat transfer fins normally attached to the outside portions of the evaporator coils.
An alternative source of heat for defrosting refrigerated display cases is the compressed vapor (“hot gas”) from the compressor. Hot gas defrost utilizes the hot gas to apply heat directly to the inside of the evaporator. Most hot gas defrost systems use the latent heat of condensation of the compressed vapor as the heat source, but some use only sensible heat of highly super heated vapor. Most hot gas defrost systems introduce the hot gas at the distributor and bypass the metering device. A defrost time control will operate the compressor during the defrost cycle and shut off the circulating air duct fans. At the same time, the control will energize the hot gas solenoid valve and allow the hot gas to enter the evaporator coil via the distributor and warm the evaporator, thus removing the buildup of frost. The availability of a portion of the energy used for hot gas defrost within the refrigeration system makes hot gas defrost attractive from an energy-saving standpoint.
Hot gas defrost is also attractive from an energy-saving standpoint because the hot gas warms the evaporator coil from within. The warm air blown across the outsides of evaporator coils by electric defrost heaters also heats up the cold case.
Traditional hot gas defrost, while attractive, has many of the drawbacks of traditional electric heater defrost. Defrosting several cold cases at the same time requires more hot gas—hot gas supplied by a compressor powered by electricity. As the compressor continues to run, the cost of power may be greatly increased by defrosting all the cases at the same time. The hot gas must be produced by same compressors used to provide refrigeration to other evaporators. To ensure sufficient head pressure for proper operation of the refrigeration system as a whole, additional controls are sometimes required.
The existing practice of defrosting a series of cold cases at the same time often leads to food spoilage and the expenses associated therewith. Shutting down the refrigeration cycles of contiguous cases at the same time can result in an increase in the temperature of the food product in the cases. Moreover, the combined effect of defrosting several contiguous cases at the same time often results in a greater increase in the display section temperature of each case.
Method and apparatus for sequentially defrosting a group of evaporators using hot gas would reduce or eliminate momentary high demands on the refrigeration system (whether for simultaneous hot gas defrost or for simultaneous cooling of multiple cases), thereby producing a more nearly constant load on the refrigeration system, reducing spoilage related to excessive defrosting, save energy, and permit use of smaller refrigeration systems while ensuring sufficient cooling capacity.